In an optical disk called rewritable disk, guided groove is provided thereon, a focussed light spot is guided along this guide and optical data is recorded and reproduced on the optical disk.
The surface of the rewritable optical disk is divided in two regions, one is a groove track, and the other is a land track. The groove track means the top surface of the guiding groove, and the land track means a bottom surface between the adjacent guiding grooves. In the rewritable disk, the information is recorded on any one of the land and groove tracks selected at will. When the information is recorded and reproduced on the groove track for an instance, an objective lens mounted on an optical head is so controlled that the spot of a laser light is in focus on the groove track and traces its center line. In order to accomplish the aforementioned servo control for the objective lens, it is indispensable to discriminate whether the focussed spot is on the land track or the groove track. In the conventional rewritable optical disk system, this discrimination is carried out by the aid of a total reflected light from the optical disk, because the total reflected light varies in a sinusoidal fashion around a certain constant as the focussed spot moves in the tracking direction, if the widths of the land and groove tracks are different from each other. In other words, the aforementioned method is not successful in case that the widths of both the land and groove tracks are the same, because the total reflected light becomes independent of the position of the focussed spot.
If the rewritable optical disk system, which can discriminate whether the focussed spot is on the land track or the groove track in case that the widths of both the land and groove tracks are the same, these tracks can be used as information-recording tracks, and data stored in the rewritable optical can be doubled.